I've got a 71 F100 2WD with the original fuel tank behind the seat. It has an electric fuel pump mounted under the driver on the frame rail. I've been getting it sorted back out after having it off the road for about six months. It's been back on the road for a couple weeks, and I've driven off about two tanks of gas.

Last night, leaving Lowes I was at a stop sign on a decline with the nose maybe pointing down 8-10 degrees (shot in the dark on degrees). As I started to pull away, it sputtered and died. After a test, I saw that the fuel pump was running but not pumping fuel. With a bit of work with the mighty-vac, I had fuel and was on the road again.

Despite showing a 1/4 tank on the fuel gauge, I thought I was nearly out of gas. So, I went immediately to the gas station. I ended up putting 13 gallons of fuel in.

- From what I've read, don't they have 19 gallon tanks?
-Why would being on that slight of a slope cause it to starve the fuel pump?

I've been considering putting in a hand pump inline under the hood, because I've had this happen a couple times. Is this just "one of those things", where you shouldn't let them get that low on gas, or is there something I need to tend to?

-Clif

 

It's been years since I've had a stock tank, so can't say the amount of fuel they hold anymore. I'm not thinking they are 19 gal?.

Nose down shouldn't affect it as the tank is very narrow. Only if it was leaning to the passenger side really bad would the pickup in the tank maybe get away from the fuel.

You can remove the sending unit easily. Take a flashlight and look down into the tank, you should see the pickup tube. Make sure it is near the bottom. Maybe it is short for some reason?

Have you replaced ALL the rubber lines? I've seen more issues with old rubber lines that are swelled shut(or have a little flapper inside) then anything else.

 

Actually, I haven't replaced the one from out of the tank that goes out of the cabin Other than that, all new lines. I'll take a look at that line.

 

You may have a leak in the fuel pickup line. When the gas gets down to that level it is not submerged anymore and begins to suck air.

 

C1TZ-9002-K .. In Cab Fuel Tank-18 gallons / Replaced by FoMoCo in 1970 with D0TZ-9002-A for use as a service part replacement.

Original applications: 1961/67 F100/1100 / 1968/69 F100/750.
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D0TZ-9002-A (replaced C1TZ-9002-K) .. In-Cab Fuel Tank-19.5 gallons / Reproduced by Blue Oval Truck Parts.

Original applications: 1970/72 F100/750 (except 1970/72 F100 with Evaporative Emission) / Retrofit: 1961/67 F100/1100 & 1968/69 F100/750.
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The fuel sending unit float is made from two pieces of copper soldered together. Solder breaks down, gas seeps in, float settles towards the bottom of the tank.

 

Well, it happened again but in a way that went against my two primary indicators. After driving about two miles, sitting at a stoplight on perfectly level ground with a full tank of gas it died. Same fix - no fuel moving. A quick hit with the mightyvac to prime the pump, and I was up and running again.

I'm planning on replacing the fuel line from the tank to the bottom of the cab this weekend.

It's just kind of flooring me it has happened now. I've had it running again for about two weeks, and have driven it quite a bit. The only two things I've changed since I was driving it successfully was adjust the timing a bit and the idle. I can see where it would die if the idle wasn't set correctly. But I can't see how the timing and idle could make the electric pump loose it's prime.

 

Yeah my 71 has an 18 gallon tank.

 

Check all your rubber gas lines, many people forgot the rubber line attaching the tank hard line & the frame hard line together that can suck air causing the engine to die.

Orich

 

Well, I changed the one hose I haven't changed out - the one from the tank to the hard line that goes out of the cabin. I could slightly smell gas before the change.

Now, the age-old automotive quandary - when you attempt to fix a problem that is intermittent and you couldn't replicate, how do you know your fix actually worked?

I let the pickup run for an hour, revving up sometimes and taking it around the block a couple times. It never stalled out. I'll keep a screwdriver and vacuum pump in it for a while, but so far, so good.

 

If the gas line showed cracks in it or brittle to bend most likely that will cause your problem. Then when they get cracked enough they start to leak.

Orich

 

If the hose isn't bent too bad that you can't see through it, look closely inside. There could be a little rubber flapper that would randomly get sucked shut by the pump, thus closing it off and making it stall. Once the vacuum from the pump bled off, it would allow it to draw fuel again.

I hope it is fixed.

 

Well, everything from the gas tank to the carb is completely new now. I ran 3/4 of a tank trouble-free. Near 1/4 tank mark it cutout. Then again a few miles later.

The fuel intake inside the tank is welded in and non-serviceable. I haven't read a whole lot about tanks themselves going bad. I know that's a possibility, but I'm kicking around ideas for anything else that may be a problem.

1. My gas cap has a hole drilled in it, so the fuel system does not draw a vacuum. I'm not really sure how or why this would make a difference. But, could that be a problem?

2. I've got a run-of-the-mill $35 fuel pump with an inline metal fuel filter screwed into one end I got from Autozone replacing the sketchy electric fuel pump on it when I got it. Could this be in a state of failure?

scratching head...

 

It's got to be a gremlin. Man that's got to so frustrating an yet seems so easy to sort out.

1.New tank
2.New fuel pump
3.New filter
4.All rubber lines new
5.Vented cap
6.=Gremlin still hiding!

So now what could cause it to suck air ?
Is your truck setup for the in frame tank that has the two way tank valve selector valve just below the drivers seat in the floor that could be the problem?

Any smell of gas in the cab when any of this has happen when it poops out ?

Orich

 

The only thing on your list that's incorrect is "new tank". It's the original behind the seat tank. But, I've made the assumption that this has never happened before and it seems like tank failure is uncommon, then the tank is not it.

I did sometimes smell some gas, but diagnosed that coming from the fuel gauge sending unit (have replaced that twice) and a couple of the screwholes holding that seem to be near the point of being stripped out. I've tightened those and added some more sealant there.

The only thing I noticed the last time it died was I could hear the fuel sloshing around behind me.

Are there any fuel pumps that are less sensitive to loosing their prime?

The other thing I've thought about is to add a manual marine-style bulb pump inline in the cab where the fuel line runs by the seat belt. If I loose prime, I could literally reach back with my left hand and get it reprimed on the fly.

But, in a perfect world, I would like to find and fix the problem.

Of course, if I wanted a perfectly-running reliable pickup that does exactly what you'd expect it to, there are plenty of those on new carlots. And what fun is that?

 

To my knowledge electric fuel pumps should be only used to pressure and not draw. The primer bulb will help of course. That is why all fuel injected vehicles have pumps in the tank. There are electric pumps that are used in boats that may work better than the auto store version. They are not cheap and all electric pumps should be wired through an oil pressure switch in case of a "flood over" or an accident.